Specifications of the LTE (Long Term Evolution) system and LTE-Advanced system are under investigation in 3GPP (3rd Generation Partnership Project).
In Japan, a specification of LTE is defined as the LTE Rel '8, and the service has started in December, 2010.
Also, the specification of the LTE Rel '9 is being defined with additional functions of MBMS (Multimedia Broadcast Multicast Service). A specification of the LTE-Advanced system, which is an advanced version of the LTE system, has been investigated as LTE Rel '10.
Moreover, investigation of the LTE Rel '11 has been also started. As one of the technologies that are considered to be introduced into the LTE Rel '11, CoMP (Coordinated Multi-Point transmission and reception) technology is known (see, for example, Non-Patent Document 1). In the following, coordinated multi-point transmission and reception will be referred to as “CoMP transmission”.
CoMP transmission will be described below.
CoMP transmission is a technology for a plurality of base stations to execute coordinated transmission and reception for communications between a user terminal and the plurality of base stations. CoMP is also called “coordinated communication”. CoMP and coordinated communication means the same unless otherwise stated, hereafter.
It is also known that objects of CoMP transmission include network MIMO, SDM (Spatial Division Multiplex), ICIC (Inter-Cell Interference Coordination or Inter-Cell Interference Cancelation) (see, for example, Non-Patent Document 2).
Therefore, depending on an object of CoMP transmission, the following embodiments are under investigation.
For example, for downlink transmission, JP (Joint Processing) and CS/CB (Coordinated Scheduling/Coordinated Beamforming) are under investigation. For uplink transmission JR (Joint Reception) and CS (Coordinated Scheduling) are under investigation (see, for example, Non-Patent Document 3).
With JP, data is transmitted from a plurality of base stations. With JP, JT (Joint Transmission), DCS (Dynamic Cell Selection), and the like are considered.
With CS/CB, for example, scheduling for transmission from a terminal is coordinated between a first base station and a second base station where the first base station covers a certain cell, and the second base station covers a cell adjacent to or overlapping with the cell covered by the first base station, and the terminal is positioned around an edge of one or more of the cells.
Interference can be reduced with coordinated scheduling or with coordinated forming of an antenna beam (called “beam”, “beamforming”, hereafter). The former is called “CS” and the latter is called “CB”.
JR includes technologies for executing coordinated reception of a radio wave from a user terminal (transmission side) at a reception side configured with a plurality of base stations, or for executing the same reception method.
For example, JR includes a technology that executes MIMO reception at a plurality of base stations via a network, or executes reception diversity among base stations.
DCS transmits data from one transmission point in a CoMP Cooperating set at a certain time. A CoMP Cooperating set is a set of transmission points from which the same or different data is transmitted at the same time. DCS is one of the methods for interference control or ICIC.
Specifically, data is transmitted from a base station to a user terminal at time T, and data is transmitted from another base station to the same user terminal at time T+1. Namely, the other base station may transmit data to another user terminal at time T+1. By separating transmitting base stations by time in this way, it is possible to reduce interference.
JT transmits the same or different data at the same time to a user terminal from a plurality of transmission points. A set of transmission points is called a “CoMP cooperating set”. Also, coordinated transmitting of data among transmission points improves reception characteristics at a user terminal. Also, precoding may be executed to reduce interference from another user terminal.
If different pieces of data are transmitted from a plurality of base stations, respectively, it is called Network MIMO. Network MIMO can improve downlink transmission speed. Also, if the same data is transmitted, downlink transmission quality can be improved.
JT is under investigation to be used for communication between sectors that are covered by a base station, or communication between different base stations.
CoMP transmission described above differs from macro diversity in general. Macro diversity in general includes transmission diversity between base stations.
1) W-CDMA, DHO (Diversity Handover), which is adopted for downlink data transmission in a wireless communication system such as PDC or the like, SHO (Soft Handover), and macro diversity assume that the same data is transmitted to a plurality of base stations. Based on the assumption that the same data is transmitted to a plurality of base stations, the same data is distributed to the plurality of base stations from an upper apparatus in advance.
Therefore, it is not necessary for the base stations to transmit (transfer) the data to be transmitted with each other.
On the other hand, with CoMP transmission, data is transmitted only to a serving base station from an upper apparatus. Therefore, the serving base station needs to transfer the data to non-serving base stations.
2) The upper apparatus does not transmit the data to the non-serving base stations. Usually, for downlink data transmission, data is transmitted to a base station via an upper apparatus. If the data is transmitted from the upper apparatus to the base station without an error, the data transmitted to the base station is deleted at the upper apparatus. Therefore, the data transmitted from the upper apparatus to the base station without an error cannot be transmitted to the non-serving base stations. Therefore, for downlink CoMP transmission, the serving base station needs to transmit the downlink transmitting data to the non-serving base station.3) If considering to use macro diversity and the like, it is a system wide selection whether to use the macro diversity. Namely, base stations cannot determine whether to use the macro diversity individually. On the other hand, with CoMP transmission, an individual base station can determine whether to use CoMP transmission.
Also, in the LTE system, two types of CPs (Cyclic Prefixes), normal CP and extended CP, are defined (see, for example, Non-Patent Document 4).
Specifically, for example, a downlink normal CP is calculated as follows.
Normal Cyclic PrefixTcp=160×Ts (1st symbol)Tcp=144×Ts (2nd symbol to 6th symbol)
Extended Cyclic PrefixTcp=512×Ts (1st symbol to 6th symbol)
here, “Tcp” represents a CP length. CP length may be represented by time.Ts=1/(2048×Δf)=32.552 ns
Therefore, if Δf=15 kHz, the normal CP is 5.21 μs or 4.69 μs, and the extended CP is 16.67 μs. Here, in the following description, although it is assumed that the normal CP is 4.69 μs for the sake of simplicity, it is substantially the same as 5.21 μs.
An extended CP is set longer than a normal CP in terms of time to prevent communication quality from degrading when propagation delay increases. Therefore, it is used with a large cell radius in the suburbs or the like, or with MBSFN (Multicast-Broadcast over Single-Frequency Network), which transmits MBMS data and receives and combines a lot more transmission waves from a plurality of base stations. Also, whether to use the normal CP or the extended CP is notified by the base station as system information. For example, in an LTE system, communication is specified in units of subframes in which a subframe transmitting data for MBSFN and a subframe transmitting normal data are used with time-division multiplexing. To make it possible for a user terminal to receive and combined MBSFN subframes transmitted from a plurality of base stations, the extended CP is used. When transmitting a MBSFN subframe, it is notified to a user terminal whether it is a MBSFN subframe in advance, the user terminal can identify that it is a normal CP or an extended CP. Also at the transmission side, a selected subframe can be transmitted as a MBSFN subframe for MBMS data with an extended CP. In this way, the CP to be used can be controlled.
Here, in an LTE system, a wireless frame of 10 ms is partitioned into 20 slots (0.5 ms), and two slots constitute one subframe (1.0 ms).